Particle size-dependent carbon source capture and carbon allocation pathways by activated sludge

Capturing organic carbon source from municipal wastewater by activated sludge is a key strategy to support energy self-sufficiency in treatment plants. In practical applications, the carbon source capture efficiency of activated sludge is often limited. Variations in particle size can lead to differences in microbial activity and metabolic pathways within activated sludge. These changes further affect the content and composition of extracellular polymeric substances (EPS), ultimately influencing its capacity to capture carbon sources. Mixed activated sludge was separated into fractions with particle sizes of 500, 250, 176, 133, and 88 μm to examine how particle size affects the capture efficiency of different forms of carbon sources. It was observed that carbon source capture performance was strongly influenced by sludge particle size, with smaller particles showing higher capture capacity. The activated sludge with a particle size of 88 μm achieved a tCOD capture efficiency of 65.95%, which is 35.95 percentage points higher than that of the 500 μm group. Adsorption and intracellular storage played dominant roles in the capture pathways. With decreasing particle size, hydrophobic interactions increased, while electrostatic repulsion weakened, lowering the energy barrier for microbial aggregation with carbon substrates. Flow cytometry and fractal analysis suggest that smaller sludge fractions contain more viable bacteria and form flocs with more irregular structures and larger surface areas. This study quantitatively links five sludge particle-size fractions with the capture of different COD forms and their carbon allocation pathways, and identified 88-133 μm as a favorable particle-size range for carbon source capture under the tested conditions. These findings provide a mechanistic and operational basis for regulating sludge particle-size distribution toward optimizing front-end carbon capture in wastewater treatment plants.